Magnetic memory cell

ABSTRACT

A magnetic memory cell includes a substrate, a transistor, a first dielectric layer disposed on the substrate, a landing pad in the first dielectric layer, a second dielectric layer covering the first dielectric layer and the landing pad, a memory stack in the second dielectric layer, and a source line in the first dielectric layer. The first dielectric layer covers the transistor. The landing pad is situated in a first horizontal plane and is coupled to a drain region of the transistor. The memory stack has a bottom electrode connected to the landing pad and a top electrode electrically connected to a bit line. The source line is situated in a second horizontal plane and is connected to a source region of the transistor. The second horizontal plane and the first horizontal plane are not coplanar.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No. 16/546,277 filed Aug. 20, 2019, which is included in its entirety herein by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to the field of semiconductor devices, and more particularly to a spin-transfer torque magnetoresistive random access memory (STT-MRAM) structure and a method of fabricating the same.

2. Description of the Prior Art

As known in the art, spin-transfer torque magnetoresistive random access memory (STT-MRAM) is a non-volatile memory that has come under much scrutiny recently in the industry, which has several advantages over the conventional magnetoresistive random access memory. For example, these advantages include higher endurance, lower-power consumption, and faster operating speed.

In a magneto-tunnel junction (MTJ) including two ferromagnetic layers having a thin insulating layer therebetween, the tunnel resistance varies depending on the relative directions of magnetization of the two ferromagnetic layers. A magnetoresistive random access memory may be a semiconductor device where magnetic elements (MTJ elements) having MTJs utilizing a tunnel magneto resistance (TMR) effect are arranged in a matrix form as a memory cell.

In conventional designs, the source lines (SL) of the MTJ bit cell arrays are arranged to be parallel to the bit line (BL). However, in conventional designs there is no direct and parallel overlap between the source line (SL) and bit line (BL) due to via and metal spacing rules. Therefore, the minimum bit cell size of conventional designs cannot be reduced or minimized as a result of metal and via spacing rules.

Because the memory includes hundreds of thousands of cells, even small area savings in each cell can result in major advantages in density of the memory. Accordingly it is highly desirable to provide apparatus and a method of improving the density of MRAM cells in a memory array by reducing the area of individual MRAM cells.

SUMMARY OF THE INVENTION

The present disclosure provides an improved spin-transfer torque magnetoresistive random access memory (STT-MRAM) structure and a method of fabricating the same that solve the above-mentioned deficiencies of the prior art.

One aspect of the disclosure discloses a magnetic memory cell comprising a substrate having an active area surrounded by an isolation region, wherein the active area protrudes from an upper surface of the isolation region. A select transistor is disposed on the active area of the substrate. A first dielectric layer is disposed on the substrate, wherein the first dielectric layer covers the active area and the select transistor. A landing pad is disposed in the first dielectric layer, wherein the landing pad is located in a first horizontal plane and electrically connected to a drain region of the select transistor. A second dielectric layer is formed to cover the first dielectric layer and the landing pad. A memory stack is disposed in the second dielectric layer, wherein the memory stack includes a bottom electrode electrically coupled to the landing pad, a top electrode electrically coupled to the bit line on the second dielectric layer, and a magneto-tunnel junction between the bottom electrode and the top electrode. A source line is disposed in the first dielectric layer, wherein the source line is located in a second horizontal plane and is electrically connected to a source region of the select transistor. The second horizontal plane and the first horizontal plane are not coplanar.

According to an embodiment of the invention, the landing pad is located in M1 layer. According to an embodiment of the invention, the source line may be located in M0 layer and adjacent to the landing pad, but is not limited thereto. According to an embodiment of the invention, the M0 layer is a tungsten layer, and the M1 layer is a damascened copper layer.

According to an embodiment of the invention, the second dielectric layer may comprise a nitrogen-doped silicon carbide (NDC) layer, a silicon oxide layer, and an ultra-low dielectric constant (ULK) material layer. The magnetic memory cell further includes a via plug disposed in the silicon oxide layer and the NDC layer. The via plug electrically connects the bottom electrode with the landing pad. For example, the via plug may be a tungsten via plug.

According to an embodiment of the invention, a word line is further disposed on the substrate and electrically connected to a gate of the select transistor, wherein the word line extends along a first direction.

These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing a partial layout of a magnetic memory device according to an embodiment of the invention.

FIG. 2 is a schematic cross-sectional view taken along line I-I′ of FIG. 1.

FIG. 3 is a schematic cross-sectional view taken along line of FIG. 1.

FIG. 4 is a schematic cross-sectional view showing a magnetic memory cell structure according to another embodiment of the present invention.

DETAILED DESCRIPTION

In the following detailed description of the disclosure, reference is made to the accompanying drawings, which form a part hereof, and in which is shown, by way of illustration, specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention.

Other embodiments may be utilized and structural, logical, and electrical changes may be made without departing from the scope of the present invention. Therefore, the following detailed description is not to be considered as limiting, but the embodiments included herein are defined by the scope of the accompanying claims.

Please refer to FIG. 1 to FIG. 3. FIG. 1 is a schematic diagram showing a partial layout of a magnetic memory device according to an embodiment of the invention. FIG. 2 is a schematic cross-sectional view taken along line I-I′ of FIG. 1. FIG. 3 is a schematic cross-sectional view taken along line II-II′ of FIG. 1. According to an embodiment of the present invention, as shown in FIG. 1 and FIG. 2, the magnetic memory device 1 comprises a substrate 10, for example, a P-type silicon substrate, but is not limited thereto. According to an embodiment of the present invention, a P-type well (PW) may be disposed in the substrate 10, but is not limited thereto. The substrate 10 comprises a memory region MA. In the memory region MA on the substrate 10, memory cells 100 arranged in an array are provided. On the substrate 10, strip-shaped and mutually parallel active areas 101 are separated from each other by strip-shaped shallow trench isolation (STI) regions 102. In accordance with an embodiment of the invention, the strip-shaped active area 101 and the strip-shaped STI region 102 extend along a reference X-axis.

As shown in FIG. 3, according to an embodiment of the present invention, the upper end of the active area 101 may protrude from the upper surface 102 a of the STI region 102 by further recess etching the STI region 102. According to an embodiment of the present invention, for example, the width AA of the upper surface 101 a of the active area 101 is approximately 70 nm, and the height of the active area 101 above the STI region 102 is approximately 35 nm, but is not limited thereto.

According to an embodiment of the invention, exemplary word lines WL₀˜WL₅ extending along a reference Y-axis are further provided on the substrate 10. According to an embodiment of the invention, the word lines WL₀˜WL₅ may be polysilicon word lines, but are not limited thereto. According to an embodiment of the present invention, as shown in FIG. 1, the magnetic memory device 1 further comprises select transistors 200 located at intersections of the word lines WL₀, WL₁, WL₃, and WL₄ and the active areas 101, respectively. According to an embodiment of the invention, each of the select transistors 200 may comprise a gate 201, a drain doping region (or drain region) 202, and a source doping region (or source region) 203. The portions where the word lines WL₀, WL₁, WL₃, and WL₄ overlap with the active areas 101 are the gates 201 of the respective select transistors 200. According to an embodiment of the invention, the exemplary word lines WL₂, WL₅ can be used to isolate signal interference. According to another embodiment of the invention, the exemplary word lines WL₂, WL₅ can be omitted, and STI region 102 can be used to isolate signal interference. According to an embodiment of the invention, the drain doping region 202 and the source doping region 203 are respectively formed in the active area 101 on both sides of the gate 201. For example, the drain doping region 202 and the source doping region 203 may be either N-type doped regions or P-type doped regions.

In FIG. 2, only the exemplary word lines WL₀, WL₁, WL₂, and two left and right symmetrical memory cells 100 a, 100 b are illustrated, wherein the memory cell 100 a includes a select transistor 200 a, and the memory cell 100 b includes a select transistor 200 b. The gate 201 of the select transistor 200 a is part of the word line WL₀, and the gate 201 of the select transistor 200 b is part of the word line WL₁. The drain doping region 202 and the source doping region 203 of the memory cell 100 a and the memory cell 100 b may be formed in a P-type well (PW) of the substrate 10. For example, the drain doping region 202 and the source doping region 203 may be N-type doped regions.

As shown in FIG. 2, multiple dielectric layers 310˜340 may be disposed on the substrate 10, but are not limited thereto. For example, the dielectric layer 310 can be an ultra-low dielectric constant (ULK) material layer. For example, the ULK material layer may be a carbon-containing silicon oxide (SiOC) layer having a dielectric constant ranging, for example, from 1 to 2.5, but is not limited thereto. According to an embodiment of the present invention, the dielectric layer 310 may be composed of a single layer of insulating material or a plurality of insulating films. The dielectric layer 310 covers the memory region MA and the select transistor 200. According to an embodiment of the present invention, the dielectric layer 320 covers the dielectric layer 310. For example, the dielectric layer 320 may include a nitrogen-doped silicon carbide (NDC) layer 321, a silicon oxide layer 322 on the NDC layer 321, and a ULK material layer 323 on the silicon oxide layer 322. For example, the silicon oxide layer 322 may be a TEOS silicon oxide layer. The TEOS silicon oxide layer refers to a silicon oxide layer deposited by using tetraethoxysilane (TEOS) as a reaction gas.

According to an embodiment of the present invention, a dielectric layer 330 and a dielectric layer 340 may be formed on the dielectric layer 320. The dielectric layer 330 may comprise, for example, a nitrogen-doped silicon carbide (NDC) layer 331 and an ULK material layer 332. The dielectric layer 340 may comprise, for example, a nitrogen-doped silicon carbide (NDC) layer 341 and a ULK material layer 342.

As shown in FIG. 1, landing pads MP are disposed in the memory region MA. The landing pads MP are arranged in an array. Each landing pad MP overlaps with the drain doping region 202 of the select transistor 200. Further, each of the landing pads MP may partially overlap with the underlying word lines WL₀, WL₁, WL₃, and WL₄ in the X-axis direction. Further, each of the landing pads MP may further partially overlap with the source doping region 203 of the select transistor 200 in the X-axis direction. In FIG. 2, only two exemplary landing pads MP₁ and MP₂ of the memory cells 100 a and 100 b are illustrated for the sake of simplicity. According to an embodiment of the invention, the landing pads MP₁ and MP₂ are disposed in the dielectric layer 310.

According to an embodiment of the invention, as shown in FIG. 2, the landing pads MP₁ and MP₂ are located in a first horizontal plane and are electrically coupled to the drain doped regions 202 of select transistors 200 a and 200 b, respectively. According to an embodiment of the invention, the landing pads MP₁ and MP₂ are located in a first metal (M1) layer. According to an embodiment of the invention, the M1 layer is a damascened copper layer. The landing pads MP₁ and MP₂ are very close to each other. For example, the distance w₁ between the landing pad MP₁ and the landing pad MP₂ may be about 60 nm, but is not limited thereto. The landing pad MP₁ and landing pad MP₂ can be electrically coupled to the drain doped regions 202 of the select transistors 200 a and 200 b via contact plugs C₁ and C₂, respectively. For example, the contact plugs C₁ and C₂ may be tungsten plugs. According to an embodiment of the present invention, contact pads CP₁ and CP₂ may be provided between the landing pad MP₁ and the contact plugs C₁ and between the landing pad MP₂ and C₂, respectively. The contact pads CP₁ and CP₂ may be tungsten contact pads, and may be formed in the zero metal (M0) layer.

As shown in FIG. 1, cylindrical memory stacks MS are disposed in the memory region MA. The cylindrical memory stacks MS are arranged in matrix or array, and the cylindrical memory stacks MS are aligned with the landing pads MP. FIG. 2 illustrates via plugs VP₁ and VP₂ disposed in the silicon oxide layer 322 and the NDC layer 321 in accordance with an embodiment of the present invention. FIG. 2 further illustrates two exemplary cylindrical memory stacks MS₁ and MS₂ disposed in the second dielectric layer 320 in accordance with an embodiment of the present invention. According to an embodiment of the present invention, the cylindrical memory stacks MS₁ and MS₂ may comprise bottom electrodes BE₁ and BE₂ electrically coupled to landing pads MP₁ and MP₂ through via plugs VP₁ and VP₂, respectively, and top electrodes TE₁ and TE₂ electrically coupled to the bit line BL₂ disposed in the third dielectric layer 330 through vias V₁ and V₂, respectively.

According to an embodiment of the present invention, as shown in FIG. 2, the via plug VP₁ is electrically connected to the bottom electrode BE₁ and the landing pad MP₁, and the via plug VP₂ is electrically connected to the bottom electrode BE₂ and the landing pad MP₂. According to an embodiment of the present invention, the via plugs VP₁ and VP₂ may be tungsten via plugs, but are not limited thereto. According to an embodiment of the invention, the bit line BL₂ and the vias V₁ and V₂ may be dual-damascened copper metal structures formed in the third dielectric layer 330.

As shown in FIG. 2, the cylindrical memory stack MS₁ further comprises a magneto-tunnel junction element MTJ₁ interposed between the bottom electrode BE₁ and the top electrode TE₁, and the cylindrical memory stack MS₂ further comprises a magneto-tunnel junction element MTJ₂ between the bottom electrode BE₂ and the top electrode TE₂. According to an embodiment of the present invention, a sidewall spacer SP₁ may be disposed on a sidewall of the cylindrical memory stack MS₁, and a sidewall spacer SP₂ may be disposed on a sidewall of the cylindrical memory stack MS₂. According to an embodiment of the present invention, for example, the sidewall spacer SP₁ and the sidewall spacer SP₂ may be a silicon nitride sidewall spacer, but are not limited thereto.

According to an embodiment of the present invention, the bottom electrodes BE₁ and BE₂ may include, for example but not limited to, tantalum (Ta), platinum (Pt), copper (Cu), gold (Au), aluminum (Al), or the like. The multi-layer structure of the magneto-tunnel junction elements MTJ₁ and MTJ₂ is well-known in the art, so the details thereof will not be described herein. For example, the magneto-tunnel junction elements MTJ₁ and MTJ₂ may include a fixed layer, a free layer, and a capping layer, but are not limited thereto. The fixed layer may be composed of an antiferromagnetic (AFM) material, for example, iron manganese (FeMn), platinum manganese (PtMn), lanthanum manganese (IrMn), nickel oxide (NiO), etc., for fixing or limiting the direction of the magnetic moment of the adjacent layer. The free layer may be composed of a ferromagnetic material such as iron, cobalt, nickel or an alloy thereof such as cobalt-iron-boron (CoFeB), but is not limited thereto. According to an embodiment of the present invention, for example, the top electrodes TE₁ and TE₂ may include a ruthenium (Ru) layer and a tantalum (Ta) layer, but are not limited thereto.

FIG. 1 also illustrates exemplary bit lines BL₀˜BL₄ extending along the reference X-axis. According to an embodiment of the present invention, as shown in FIG. 1, the bit lines BL₀˜BL₄ approximately overlap and align with the underlying strip-shaped active areas 101. According to an embodiment of the present invention, the bit lines BL₀˜BL₄ may be formed in the upper metal interconnect, for example, in the second metal (M2) layer or in the third metal (M3) layer.

As shown in FIGS. 1 and 2, source lines SL extending along the reference Y-axis direction are further provided on the substrate 10 (FIG. 1 illustrates only SL₀ and SL₁, and FIG. 2 illustrates only SL₀). For example, the extension direction of the source lines SL is parallel to the word line WL and perpendicular to the bit line BL. As shown in FIG. 1, the source line SL₀ is disposed between the word lines WL₀ and WL₁, and the source line SL₁ is disposed between the word lines WL₃ and WL₄. As shown in FIG. 2, the illustrated source line SL₀ is disposed in the first dielectric layer 310.

According to an embodiment of the invention, the illustrated source line SL₀ is located in a second horizontal plane and is electrically coupled to the common source region 203 of the select transistors 200 a and 200 b. According to an embodiment of the invention, the second horizontal plane is lower than the first horizontal plane. In other words, the source line SL₀ illustrated in FIG. 2 is lower than the landing pads MP₁ and MP₂. According to an embodiment of the present invention, the source line SL₀ illustrated in FIG. 2 is located in the M0 layer and is in close proximity to the landing pads MP₁ and MP₂. For example, the line width w₂ of the source line SL₀ illustrated in FIG. 2 is about 40 nm, so that the gap width between the source line SL₀ and the adjacent pads MP₁ and MP₂ is about 10 nm. According to an embodiment of the invention, the M0 layer is a tungsten layer.

The main advantage of the present invention is that the size of the MRAM memory cell can be further reduced to about 0.022 μm² through the design of the magnetic memory device 1 described above, mainly because the source line SL is disposed in the M0 layer. The pitch size of the active area 101 in the reference Y-axis direction can be reduced from 240 nm to 140 nm. Further, by protruding the active area 101 shown in FIG. 3 above the STI region 102, the problem of insufficient turn-on current (I_(ON)) due to the reduction in the pitch size of the active area 101 in the reference Y-axis direction may be solved.

FIG. 4 is a schematic cross-sectional view showing a magnetic memory cell structure according to another embodiment of the present invention, wherein like elements, materials or layers are designated by like reference numerals. As shown in FIG. 4, the magnetic memory cell 100 comprises a substrate 10 having a memory region MA. A select transistor 200 is provided in the memory region MA on the substrate 10. A first dielectric layer 310 is disposed on the substrate 10, wherein the first dielectric layer 310 covers the memory region MA and the select transistor 200. A landing pad MP is disposed in the first dielectric layer 310, wherein the landing pad MP is located at a first horizontal plane and electrically connected to the drain doped region (or drain region) 202 of the select transistor 200. The second dielectric layer 320 covers the first dielectric layer 310 and the landing pad MP.

A cylindrical memory stack MS is disposed in the second dielectric layer 320, wherein the cylindrical memory stack MS comprises a bottom electrode BE electrically coupled to the landing pad MP, a top electrode TE electrically coupled to the bit line BL on the second dielectric layer 320, and magneto-tunnel junction element MTJ between the bottom electrode BE and the top electrode TE.

The difference between the magnetic memory cell 100 in FIG. 4 and the structure shown in FIG. 1 to FIG. 3 is that the source line SL of the magnetic memory cell 100 in FIG. 4 is disposed in the second dielectric layer 320. The source line SL is located at a second horizontal plane and is electrically connected to a source doped region (or source region) 203 of the select transistor 200. The second horizontal plane is higher than the first horizontal plane. According to an embodiment of the invention, the landing pad MP is located in the first metal (M1) layer. According to an embodiment of the present invention, the source line SL may be located in the second metal (M2) layer, but is not limited thereto.

Similarly, as shown in FIG. 4, the second dielectric layer 320 may include a nitrogen-doped silicon carbide (NDC) layer 321, a silicon oxide layer 322, and an ultra-low dielectric constant (ULK) material layer 323. The magnetic memory cell 100 includes a via plug VP disposed in the silicon oxide layer 322 and the NDC layer 321. The via plug VP is electrically connects the bottom electrode BE with the landing pad MP. For example, the via plug VP may be a tungsten via plug.

Another aspect of the present disclosure discloses a method of forming a magnetic memory cell, comprising: providing a substrate 10 having a memory region MA. A select transistor 200 is formed in the memory region MA on the substrate 10. A first dielectric layer 310 is formed on the substrate 10, wherein the first dielectric layer 310 covers the memory region MA and the select transistor 200. A landing pad MP is formed in the first dielectric layer 310, wherein the landing pad MP is located at a first horizontal plane and electrically connected to a drain region 202 of the select transistor 200. A second dielectric layer 320 is formed to cover the first dielectric layer 310 and the landing pad MP. A cylindrical memory stack MS is formed in the second dielectric layer 320, wherein the cylindrical memory stack MS includes a bottom electrode BE electrically coupled to the landing pad MP and a top electrode TE electrical coupled to bit line BL on the second dielectric layer 320. A source line SL is formed, wherein the source line SL is located at a second horizontal plane and is electrically connected to a source region 203 of the select transistor 200.

Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims. 

What is claimed is:
 1. A magnetic memory cell, comprising: a substrate comprising an active area surrounded by an isolation region, wherein the active area protrudes from an upper surface of the isolation region; a select transistor on the active area of the substrate; a first dielectric layer disposed on the substrate, wherein the first dielectric layer covers the active area and the select transistor; a landing pad in the first dielectric layer, wherein the landing pad is situated in a first horizontal plane and is electrically coupled to a drain region of the select transistor; a second dielectric layer covering the first dielectric layer and the landing pad; a memory stack in the second dielectric layer, wherein the memory stack comprises a bottom electrode electrically coupled to the landing pad and a top electrode electrically coupled to a bit line disposed above the second dielectric layer; and a source line in the first dielectric layer, wherein the source line is situated in a second horizontal plane and is electrically coupled to a source region of the select transistor, wherein the second horizontal plane and the first horizontal plane are not coplanar.
 2. The magnetic memory cell of claim 1, wherein the second dielectric layer comprises a nitrogen-doped silicon carbide (NDC) layer, a silicon oxide layer on the NDC layer, and an ultra-low k material layer on the silicon oxide layer.
 3. The magnetic memory cell of claim 2 further comprising a via plug disposed in the silicon oxide layer and the NDC layer.
 4. The magnetic memory cell of claim 3, wherein the via plug electrically connects the bottom electrode to the landing pad.
 5. The magnetic memory cell of claim 3, wherein the via plug is a tungsten via plug.
 6. The magnetic memory cell of claim 1, wherein the landing pad is in a first metal (M1) layer.
 7. The magnetic memory cell of claim 6, wherein the source line is in a zero metal (M0) layer and is in close proximity to the landing pad.
 8. The magnetic memory cell of claim 7, wherein the zero metal (M0) layer is a tungsten layer and the first metal (M1) layer is a damascened copper layer.
 9. The magnetic memory cell of claim 1 further comprising a word line disposed on the substrate and electrically coupled to a gate of the select transistor, wherein the word line extends along a first direction. 